Fullerenes are a family of closed-cage molecules formed entirely of carbon in the sp.sup.2 -hybridized state and constitute the third form of carbon after diamond and graphite. These spherical, cavity-containing molecules have been found to possess novel, remarkable properties, and the buckminster-fullerene C.sub.60 has been widely investigated (Hirsch; Taylor, 1993).
Fullerenes are produced in a carbon-arc process where an inert gas flows through a reaction vessel at a controlled pressure. A potential, either a.c. or d.c., is applied between two graphite rods in the vessel, where one rod can be smaller in diameter than the other. As the rods are brought close together, a discharge occurs resulting in the formation of a carbon plasma. As the smaller rod is consumed, the rods are kept at a constant distance from each other of approximately 1-5 mm. The electric current depends on the size of the rods, their separation, the gas pressure, and other parameters, but typically it is about 100 A (Ebbesen).
At the extremely high temperatures in the plasma of the carbon-arc process where fullerenes are synthesized, a variety of by-products are also synthesized. The first recognized by-products were nested carbon tubes and polyhedra (Iijima), the discovery of which extended the dimensions and geometries of fullerenes into the nanometer domain.
Exohedral and endohedral modifications to fullerenes, in particular to the easily accessible C.sub.60, have been reported (Hirsch; Taylor, 1993; Heath; Chai; Johnson; Moro). Exohedral modifications of fullerenes by addition reactions have shown C.sub.60 to be reactive, and derivatives of fullerenes have been produced (Bethune, Hirsch). Endohedral modifications involve deposition of atoms into the cavity within the cage. Helium, other noble gas atoms, and metal atoms have been incorporated in fullerenes (Bethune).